包覆及未包覆奈米零價鐵對青鱂魚苗毒性效應之探討

Abstract

奈米零價鐵(Nanoscale zerovalent iron, nZVI)常被使用於現地地下水與土壤汙染復育，研究指出經表面修飾劑包覆之nZVI能夠提升其在地下水層中之移動能力與汙染物移除效率。目前關於nZVI之生物毒性及其於水體環境中宿命之研究非常有限。本篇研究目的在探討不同形式nZVI及相關奈米鐵物種對青鱂魚苗(Oryzias latipes)毒性效應及其毒性作用機制。高濃度(50、100 mg/L)之CMC-nZVI、nZVI、nFe3O4與Fe2+對青鱂魚苗之急毒性順序為Fe2+＞CMC-nZVI＞nZVI＞ nFe3O4。CMC-nZVI及nZVI在溶液中快速解離Fe2+並產生大量的ROS，使青鱂魚苗暴露在缺氧與大量ROS之逆境下造成魚苗死亡。CMC-nZVI (50、100 mg/L)藉由穩定劑之包覆使NPs穩定懸浮於暴露溶液並增加暴露有效劑量，並且在溶液中產生較多的Fe2+與活性氧物種(Reactive oxygen species, ROS)而使急毒性高於同濃度下之nZVI。穿透式電子顯微鏡(Transmission electron microscopy, TEM)影像顯示nZVI與nFe3O4易於聚集成團粒，但CMC-nZVI則否。高濃度(100 mg/L) CMC-nZVI及nZVI溶液中之DO快速地降至0 mg/L並持續3小時以上，並且使總ROS強度增加。在24小時之內，CMC-nZVI溶液之鐵物種逐漸轉變為Fe2+及Fe3+，而nZVI溶液中鐵離子濃度較低。低濃度(0.5、5 mg/L)暴露下，CMC-nZVI與nZVI在暴露過程(7天)中使魚苗體內過氧化氫酶(Catalase, CAT)之活性先下降後上升；nFe3O4與Fe2+在暴露過程(14天)中增加魚苗體內CAT、穀胱甘肽還原酶(Glutathione reductase, GR)之活性與總ROS強度並降低超氧歧化酶(Superoxide dismutase, SOD)之活性，進而對魚苗造成氧化壓力。由組織病理觀察發現，青鱂魚苗暴露CMC-nZVI、nZVI (25、100 mg/L)7天後導致鰓與腸道組織有鐵累積的現象，且腸道組織受到損害；暴露nFe3O4與Fe2+ (25、100 mg/L) 3天與nFe3O4與Fe2+ (5 mg/L)14天後亦會導致鰓與腸道組織有鐵累積的情形。因此，在本研究條件之下，穩定劑之包覆提高CMC-nZVI對青鱂魚苗之毒性效應，而CMC-nZVI與nZVI在溶液中反應後之產物(如nFe3O4與Fe2+)對水生生態系仍具有潛在性的風險。

Nanoscale zerovalent iron (nZVI) has been increasingly used for in situ groundwater and soil remediation because of its high redox activity and specific characteristics of nanoparticles (NPs). Recent applications are often involved in the use of surface-modified nZVI with the stabilizer that can enhance mobility of nZVI while travelling through the underground aquifer. However, the ultimate fate of iron NPs under- or above-ground aquifer and associated risks of exposure and toxicity to environmental life remain poorly understood at present. The objective of the study is to understand modes of toxic action of different iron NPs by comparing their lethal and sublethal effect(s) in medaka (Oryzias latipes). We have treated medaka larvae with thoroughly characterized solutions containing nZVI (uncoated), carboxymethyl cellulose sodium (CMC)-coated nZVI, nanoscale iron oxides (nFe3O4) and ferrous (Fe2+) ion at 0.5 - 100 mg/L for 7 days aqueous exposure. TEM images indicated that uncoated nZVI and nFe3O4 tended to aggregate, but CMC-nZVI did not. Dissolve oxygen suddenly dropped to 0 mg/L for >180 min and total reactive oxygen species (ROS) were elevated after the addition of CMC-nZVI and nZVI (100 mg/L). Fe2+ and Fe3+ were major species in CMC-nZVI, but at low concentration in the nZVI solution during 24 hr’s monitoring. In the meantime, the acute toxicity were in the order of Fe2+ ＞ CMC-nZVI ＞ nZVI ＞ nFe3O4 at high exposure concentrations (50 and 100 mg/L). At low exposure concentrations (0.5 and 5 mg/L), nFe3O4 and Fe2+ significantly increased CAT, GR activities and intracellular ROS levels, but decreased SOD activity. CMC-nZVI and nZVI only altered CAT activity. Based on histopathological analyses, treated groups (> 25 mg/L of CMC-nZVI, nZVI, nFe3O4 and Fe2+) led to various degrees of accumulation of iron particles in the fish gill and intestine tissues. Intestine wall and intestinal villi were damaged at some treated groups (> 25 mg/L of CMC-nZVI and nZVI). In this study, we observed that the use of stabilizer on nZVI and its reaction products (nFe3O4 and Fe2+) may increase hazardous risk of nZVI to the aquatic ecosystem.